Protective circuit

ABSTRACT

The invention relates to a protective circuit for limiting the voltage for a device to be protected, particularly a load with an upstream-connected voltage and current limiting device, the device to be protected being provided in an output circuit of the protective circuit and is separable by means of a switching element, a voltage detector being provided with an electronic switch on the one hand and a voltage sensor on the other and the control terminal of the electronic switch is in electrical operative connection with the voltage sensor. A microdevice with a control circuit is provided, in which the microdevice has the switching element and is designed with a high input resistance of the control circuit. The control circuit is galvanically separated from the output circuit switchable by means of the control circuit, in which the control circuit of the microdevice is in electrical operative connection with the electronic switch and the device to be protected for the separation of a voltage applied is located in the output circuit of the microdevice.

[0001] The invention relates to a protective circuit for limiting thevoltage for a device to be protected, particularly a load with anupstream-connected voltage and current limiting device, in accordancewith the preamble of claim 1.

[0002] Protective circuits of this type are known from EP 359 912 A1, EP310 280 B1 or German patent 36 22 268. These known protective circuitssuffer from various disadvantages. Thus, e.g. following the response ofthe corresponding fuse mechanism in EP 359 912 A1 it is necessary tomanually replace a fuse. The circuits of German patent 36 22 268 and EP310 280 B1 suffer from the disadvantage of high shunt currents and inaddition an undesired voltage drop occurs in the operating area. Anotherdisadvantage is that in the case of a fault a high power loss can arise.

[0003] Although in the not previously published PCT application WO00/62394 A1 a solution is obtained to the aforementioned problems inthat a protective circuit is placed upstream of the fuse mechanism forthe load and has a rapid response behaviour, so that in the case of anovervoltage or an excessive current a rapid switching off of thedownstream circuits is possible. However, the switch in this case isconstituted by a FET located in the longitudinal branch between an inputterminal of the protective circuit and the downstream fuse mechanism.

[0004] When using an FET the wiring possibilities and the arrangement inthe protective circuit are limited and relatively inflexible, so thatimprovements are needed.

[0005] DE 38 04 250 C1 describes a circuit arrangement for currentlimiting purposes, particularly for use in the case of digital terminaldevices. For switching off a voltage in a longitudinal branch of thecircuit arrangement is provided the source-drain path of a field effecttransistor, whose passage is controlled by its gate as a function ofboth the turning on of a transistor and the charging of a capacitor.

[0006] DE 296 13 790 U1 relates to a micromechanically manufacturedmicroswitch in which, as comparable with a reed contact, a switchingmovement can be brought about by a magnetic field acting on an elastic,bendable support.

[0007] DE 41 00 634 A1 discloses a test device for IC's, particularly inmounted printed circuit boards, in which for the connection ofindividual test points of a testpiece with different test channels aplurality of microrelays is provided.

[0008] Thus, the object of the present invention is to provide aprotective circuit for limiting the voltage of a device to be protected,particularly a load with an upstream-connected voltage and currentlimiting device, which can be flexibly designed in accordance with thecircuit requirements, which has negligible direct-axis voltage componentdrops and shunt currents and which can in particular form a suitableprotective function for a Zener barrier connected upstream of a load.

[0009] According to the invention this object is achieved by aprotective circuit having the features of claim 1.

[0010] A protective circuit according to the preamble is furtherdeveloped according to the invention in that the switching device is amicromechanical component, which has a control circuit with a high inputresistance and an output circuit galvanically separated from the controlcircuit and which is switchable by means of the control circuit, theoutput circuit in the opened switching state having a galvanicseparation of its output terminals.

[0011] Therefore a fundamental idea of the invention is not to designthe necessary switch as a bipolar transistor, FET or the like, butinstead to choose a micromechanical component with a high inputresistance and galvanic separation of the output circuit and controlcircuit and which on switching off or opening the switch has a galvanicseparation of the output terminals.

[0012] Appropriately the micromechanical component is implemented in theform of a microrelay based on materials and methods such as are of aconventional nature in micromechanics and/or semiconductor production.

[0013] In particular, in a highly advantageous manner, a choice is madeof an e.g. silicon-based microrelay, which can have a suitable layerstructure, e.g. similar to that of semiconductor components and whoselayers are structured in such a way that the switching mechanicalelement is operable by electrostatic or piezoelectric forces, i.e. bychange of charge. With respect to contact making or opening thecontacts, said silicon microrelay can operate in a similar manner to thefunction of a leaf spring, a flectional bar beam or the like. Such arelay is described in exemplified manner in U.S. Pat. No. 5,638,946.

[0014] The voltage sensor means can be constituted by a voltage sensordiode, particularly a Zener diode, a power sensor or a temperaturesensor, e.g. a temperature-variable resistor, e.g. a thermistor. In apreferred circuit design with the electronic switch (T1) is associatedat least one operating resistor (Rc) and with the voltage sensor diode(Dz) at least one resistor (Rb) and the control terminal of theelectronic switch (T1) is in operative connection with the resistor (Rb)and the voltage sensor diode (Dz).

[0015] A serious disadvantage with regards to the flexibility of thecircuit design is the galvanic separation with respect to the microrelaybetween its control circuit and the terminals of the load circuit, aswell as the high input impedance of the control circuit. Due to itsmicrostructure said microrelay can also be implemented in an integratedconstruction with the further electronics and a subassembly as a SMD orin the form of a chip is conceivable.

[0016] However, as a function of the intended use, the possibilityexists of providing external components with respect to the microrelayor to implement the necessary components directly as a subassembly withthe said microrelay.

[0017] The high vibration resistance of the microrelay, which reducesthe probability of faults is also highly advantageous. On choosing amicrorelay as the switch, great importance is attached to the fact thatit is very flexible and can be adapted in accordance with needs, i.e.with an existing voltage or current sensor it can be positioned in thecircuit with a high degree of freedom in accordance with the startingrequirements for the downstream load.

[0018] Thus, the microrelay with its control terminals can be directlycontrolled by means of a collector-emitter or collector-base ordrain-source or drain-gate path of a transistor, respectively. Theintegration of a microrelay in existing circuits can normally take placerelatively easily when there is a correct evaluation of the microrelaycharacteristics. Compared with when a FET is used as the switch, thevoltage at the output is switched off without additional wiring in orderto obtain a hysteresis in a type of tilting function. By means of aprotective circuit with a microrelay as the switch, it is possible inadvantageous manner to implement circuits against an overvoltage andoverload, also with self-holding.

[0019] The important advantage of using a corresponding microdevice ormicrorelay as the switch in a protective circuit is the significantsimplification with regards to the components needed for controlling themicrorelay. In appropriate cases this simplification can lead to themicrorelay alone and without further upstream components taking over theresponse function and the disconnection function for the downstreamload.

[0020] Thus, in simplified manner, the invention provides a detectordevice for overvoltage and overload, which as the core cell in theprotective circuit operates the switch of the corresponding microrelay.This core cell can be equipped with extension cells for adjusting orsetting parameters of the core cell. It is also possible to incorporatethe inherent protection of the microrelay against overvoltages. Such aprotective circuit structure permits a high flexibility with respect tothe design with switching transistors of the npn type or pnp type or asa FET.

[0021] As a function of requirements the microrelay can be designed as anormally open contact or as a normally closed contact.

[0022] Particularly as a result of the very short response behaviour,the high input resistance and the high switching frequency themicrorelay also permits the protection of a voltage-current limitingdevice, normally constructed as a Zener barrier and which is connectedupstream of a load or primary element. In other words a correspondingprotective circuit has a faster separation than e.g. adownstream-connected Zener barrier, so that it is also possible toprotect subassemblies in the explosion-proof range.

[0023] The invention is described hereinafter relative to schematicembodiments and the attached drawings, wherein show:

[0024]FIG. 1 Two variants of a core cell with pnp or npn transistor,respectively.

[0025]FIG. 2 An extension cell for extending the basic circuit with acore cell according to FIG. 1.

[0026]FIG. 3 The operating principle of the protective circuit accordingto the invention with core cells in two embodiments and extension cells,by means of which the different variants of the protective circuit canbe derived by the as desired use of the elements shown.

[0027]FIG. 4 An embodiment of the circuit with a core cell with npntransistor and a microrelay MR1 as the normally open contact at theoutput.

[0028]FIG. 5A further derivation of FIG. 3 with a npn transistor, acontrol terminal of the microrelay being on the transistor base insteadof the emitter as in FIG. 3.

[0029]FIG. 6 Another embodiment of the protective circuit with amicrorelay as the normally closed contact.

[0030]FIG. 7 Another variant of the core cell with pnp transistor and aZener diode parallel to the microrelay MR1 for the inherent protectionof the latter.

[0031]FIG. 8 Another embodiment of the protective circuit in which theoutput voltage is fed back across a resistor Rh as a hysteresis resistorto the core cell transistor base.

[0032]FIG. 9 The simplest embodiment of the protective circuit with amicrorelay, as is possible with corresponding parameter adaptation.

[0033]FIG. 10A protective circuit with microrelay withdownstream-connected voltage-current limiting device and following load.

[0034]FIG. 1 diagrammatically shows two variants of a voltage detector4. The voltage detector 4 which can also be called a core cell 4essentially comprises a parallel circuit, in whose one branch is locateda transistor T1 and a collector resistor Rc in series therewith. In theother branch is provided a Zener diode Dz and a base resistor Rb. Thebase of the transistor T1 is passed to the connecting point 24 betweenthe Zener diode and the base resistor Rb. In the left-hand case shownthe transistor T1 is a bipolar pnp transistor. In the right-hand variantshown a npn transistor T1′ is provided. It is also possible to use FET'sin place of bipolar transistors.

[0035]FIG. 2 shows an example of an extension cell for the core cellvariants shown in FIG. 1. In this case the extension cell comprises aZener diode Dz and a resistor Rv connected in series therewith.

[0036]FIG. 3 diagrammatically shows several variants of the protectivecircuit according to the invention. The protective circuit 1 has a corecell 4 with a pnp transistor T1 and two extension cells 5, 6 with Zenerdiodes Dz+ and Dz− and resistors Rv+ and Rv−. The core cell 4 isconnected across the extension cells 5, 6 to a positive or negativesupply voltage 11 or 12.

[0037] As the rapid switch, a microrelay MR1 is provided as the normallyclosed contact in a first advantageous variant of the protective circuit1. Said microrelay MR1 has its input 7 at the emitter of the transistorT1 and its other input 8 at the collector of transistor T1. Theswitchable output 17 is at the input terminal 11 of the protectivecircuit 1, whereas the other output 18 of the microrelay MR1 leads tothe device L1, S to be protected. The device to be protected in thefirst variant of FIG. 3 is shown in exemplified form as a load L1 withvoltage and current limiting device as a fuse mechanism S. Both the loadL1 and the fuse mechanism S have their in each case other terminal atthe protective circuit terminal 14.

[0038] The fuse mechanism S shown diagrammatically in FIG. 3 can be aZener barrier with a fuse in the serial branch to the load L1, as isshown in the following FIG. 10.

[0039] In the standard case the two diodes Dz+, Dz− and one of the tworesistors Rv+, Rv− are replaced by bridges, so that the first terminalof the core cell 4 is connected directly or across Rv+ to 11, 13. Theother terminal of the core cell 4 then leads either directly or acrossRv− to the line 12, 14.

[0040] In operation in the previously described circuit according toFIG. 3, the device to be protected in the case of an overvoltage isdisconnected with such a brief time lag by the microrelay MR1 that inthe short disconnection phase the power still transferred to the deviceto be protected is not sufficient to damage elements of said device S,L1 to be protected.

[0041] In normal operation the transistor T1 is nonconductive or blockedand the switch 21 of microrelay MR1 is closed. The supply voltage or thevoltage for the device to be protected is applied to the input terminals11, 12, because the switch 21 of microrelay MR1 is closed.

[0042] Thus, if a voltage peak reached the input terminals 11, 12, as afunction of the voltage of the Zener diode Dz the latter would becomeconductive and as a result of the current flow in the nodal point 24would bring about the switching through of transistor T1.

[0043] Thus, as a result of the voltage switched off at terminals 7 and8 of microrelay MR1, the switch 21 would open, so that there would be arapid disconnection of the device S, L1 to be protected in the case ofan overvoltage. Thus, in this way the device to be protected isprotected against an overvoltage and the response of a fuse can beprevented by the rapid opening of the microrelay MR1.

[0044]FIG. 3 further shows the extension of the basic circuit comprisingthe core cell 4 and microrelays MR1, MR2 by extension cells 5, 6. In thecase of FIG. 3, the extension cell 5 comprises a further Zener diode Dzand a series resistor Rv in series therewith. They can be providedindividually or in combination as extension cells, e.g. serially to thecore cell 4 with respect to the positive input terminal 11 and/or as anextension cell 6 with respect to the other input terminal 12.

[0045] As a result of such an extension cell 5 or 6 it is possible tolimit the current flowing through the core cell 4. However, so-to-speakan adjustment of the starting voltage of the core cell 4 can therewithalso be brought about. The core cell 4 can also have a Zener diode 26,which takes over the function of an input overvoltage protection for themicrorelay MR1 (cf. FIG. 7).

[0046] In the further variant of the protective circuit 1 shown in FIG.3 abstractly the microrelay MR2 is provided as a normally closedcontact. The rapid disconnection function in the case of an overvoltageis implemented by the microrelay MR2. Said microrelay MR2 has its inputterminals 9, 10 parallel to the collector resistor Rc of the previouslydescribed core cell 4. In the present case the microrelay MR2 isconstructed as a normally closed contact with a switch 22, which isclosed in normal operation, because a sufficient control voltage is notavailable across resistor Rc.

[0047] The output terminal 19 of microrelay MR2 is at the input terminal11 of the supply voltage and its further output terminal 20 is at thedevice L2 to be protected, which is at the terminal 12 of the protectivecircuit 1.

[0048] If an overvoltage reaches the input terminals 11, 12, once againand as hereinbefore, the Zener diode Dz becomes conductive as a functionof the voltage applied and due to the current flow in the nodal point 24brings about the switching through of transistor T1. As a result of thisin the collector-emitter circuit of T1 would flow a current causing avoltage drop at Rc. This voltage drop at the collector resistor Rc,which is at the terminals 9, 10 of the microrelay MR2, brings about anopening of switch 22 and therefore a separation of the device L2 to beprotected.

[0049] Due to the extremely short response time or pull-in time lag, thehigh input resistance, the low power consumption, the high switchingfrequency and the galvanic separation of the control and load circuits,such a microrelay MR1 or MR2 is eminently suitable as a fuse switch,particularly for overvoltages, especially since a galvanic separation ispresent at the terminals to the load after opening the switch.

[0050] The response value for an overvoltage to be detected isestablished by the Zener diode. However, if the microrelay responsevoltage is above the input voltage available at 11, 12, the microrelaycannot be controlled without additional measures.

[0051] Thus, for achieving very low response levels, it can beappropriate to connect upstream of the microrelay MR1 of MR2 a voltagetransformer, which transforms the low level to a corresponding startingvoltage level. The voltage transformer can ideally be constructed as aninternal charge pump in the microrelay.

[0052] As is shown by the variants of the embodiment according to FIG.3, as a function of whether the microrelay is constructed as a normallyclosed or normally open contact, solely by an appropriate insertion ofthe microrelay in the protective circuit 1 a separation can be achieved.As a result of the high input resistance account need not be taken ofthe microrelay as an operating resistor and instead it can be added as aswitching element at virtually any suitable location substantiallyparallel to the existing components of the core cell 4.

[0053] As the core cell 4 according to FIG. 3 can be varied in differentways together with the extension cells 5, 6, in the following FIGS. 4 to8 are shown embodiments together with the corresponding connectionposition of the microrelay MR1 or MR2.

[0054] In FIG. 4 the core cell 4 comprises Zener diode Dz in series withthe base resistor Rb. In the parallel branch in FIG. 4 is provided a npntransistor T1′ and a collector resistor Rc. As the extension cell 5 aseries resistor Rv is provided facing the input terminal 11.

[0055] The microrelay MR1, which in the example of FIG. 4 is a normallyopen contact, has its terminal 7 at the collector of transistor T1′ andits other terminal 8 at the input terminal 12 or at terminal 14,respectively. The output 17 of microrelay MR1 is at terminal 11. Theother output 18 is provided for the terminal of a device LI to beprotected, optionally together with a fuse mechanism S, which with itsother terminals would be located at output 14.

[0056] Normally the microrelay MR1 is closed, so that the correspondingsupply voltage is at the terminals 18, 14 relative to the device to beprotected. If an overvoltage reaches terminals 11, 12, then thetransistor T1′ is switched through. The microrelay MR1 opens andconsequently switches off in a very short time the corresponding deviceto be protected or the upstream fuse mechanism S1 (FIG. 1),respectively, so that damage to the load at terminals 18, 14 is avoided.

[0057] An embodiment is also conceivable, in which the device L1, S1 tobe protected has its one terminal at terminal 11 and its other terminalis connected to the output 17 of microrelay MR1. Terminal 18 ofmicrorelay MR1 would then be connected to line 12, 14.

[0058] Another advantageous embodiment is shown in FIG. 5. The core cellcomprises Zener diode Dz and the series-connected base resistor Rb,which is followed by a series resistor Rv as the extension cell 6 facingthe input terminal 12.

[0059] The second branch of the core cell 4 has a npn transistor T1′,whose collector is located across the collector resistor Rc at input 11.The emitter of the transistor T1′ is at the connecting point to the baseresistor Rb and series resistor Rv. The base is at the nodal pointbetween the Zener diode Dz and resistor Rb. The microrelay MR1constructed as a normally open contact has its terminal 7 at thecollector and its terminal 8 at the base of transistor T1′. The outputterminal 17 is at the input 11. The other output terminal 18 serves as aconnection point for the device to be protected, whose further terminalis at output 14.

[0060] In a comparable manner the protection against an overvoltagecomprises the microrelay MR1 opening when such an overvoltage occurs andas a result the following load is switched off.

[0061]FIG. 6 shows the protective circuit with a microrelay MR2 as anormally closed contact. The core cell 4 comprises a Zener diode Dz anda base resistor Rb in one branch. A npn transistor T1′ with a collectorresistor Rc is provided in the other branch. The core cell 4 isconnected across a series resistor Rv to the input terminal 11. The baseresistor Rb and the emitter of transistor T1′ are located at the inputterminal 12 or the output 14, respectively.

[0062] The terminal 9 of microrelay MR2 is at the collector resistor Rcor series resistor Rv, respectively. The other terminal 10 is at thecollector. The first output 19 of microrelay MR2 is at the inputterminal 11. The second output 20 of microrelay MR2 serves as aconnection point for a load, whose other terminal would be at output 14.If an overvoltage occurs at the input terminals 11, 12, a current flowsacross Zener diode Dz to the base of transistor T1′, which switchesthrough, so that the potential at the collector resistor Rc reaches theterminals 9, 10 of microrelay MR2, which immediately opens andconsequently separates and protects the load at the terminals 20, 14.

[0063] In the embodiment of FIG. 7 a further wiring possibility is shownfor a microrelay MR1 as a normally open contact. The core cell 4 isslightly modified and in the left-hand, parallel branch a base resistorRb is in series with a Zener diode Dz.

[0064] Connected downstream of the Zener diode Dz is provided a seriesresistor Rv in the sense of an extension cell 6 facing the inputterminal 12 with negative potential. In the right-hand branch isprovided a pnp transistor T1 with its emitter at the input terminal 11and its collector across a collector resistor Rc at input 12 or outputterminal 14, respectively.

[0065] The microrelay MR1 has one terminal 7 at input terminal 11 andits other terminal 8 at the collector of-the transistor T1. The output17 of microrelay MR1 is also at the supply voltage input terminal 11,whereas the other terminal 18 leads to a not shown load, whose otherterminal would be located at the output terminal 14. For the inherentprotection of the microrelay MR1 against an excessive control voltage, aZener diode 26 is located parallel to the microrelay MR1 or to theemitter-collector path of the transistor T1, respectively, with itsanode at the collector or the terminal 8 of the microrelay MR1,respectively, and with its cathode at the input terminal 11. With saidwiring on the one hand a connected load or a fuse mechanism,respectively is protected in the case of an overvoltage by the openingof the microrelay MR1. Simultaneously with a high voltage the microrelayMR1 would be protected by the Zener diode 26, because the latter in sucha case would limit the control voltage.

[0066] A circuit design with a feedback of the potential at the output18 of microrelay MR1 to the base or nodal point 24 of the transistor T1is shown in FIG. 8. The core cell 4 with extension cell 6 corresponds tothe embodiment of FIG. 3. The output 18 of the microrelay MR1constructed as a normally open contact is in the embodiment of FIG. 8fed back across a hysteresis resistor Rh to the nodal point 24. By meansof this feedback of the potential at output 18 and which is alsosupplied to the load, a stable disconnection of the microrelay MR1extending to self-holding is obtained. However, the hysteresis functiononly exists in the case of a connected load with upstream-connectedZener barrier as the voltage and current limiting device.

[0067] As a result of the construction and switching behaviour of themicrorelay the advantage arises that in the case of its design as anormally closed contact, as shown in FIG. 9, the microrelay MR2 can beused, without further component expenditure, as a protective circuit.However, the prerequisite is that the starting voltage or overvoltageoccurring at terminals 9 and 10 against which the following load atterminals 20, 14 is to be protected corresponds to the starting voltageof microrelay MR2. In the embodiment according to FIG. 9 the terminal 9and output terminal 19 of microrelay MR2 are at the input terminal 11.The output 10 is at the input terminal 12 or output terminal 14,respectively.

[0068] A load to be protected or a fuse mechanism S according to FIG. 3would correspondingly be connected between the terminals 20, 14. If inthe embodiment according to FIG. 9 an overvoltage reached the microrelayMR2, with an extremely short switching time via its switch themicrorelay would separate the terminals 19, 20 and would correspondinglyprotect the following load against an overvoltage.

[0069] As a result of the construction of the microrelay based onsemiconductor materials and also in integrated form, it is also possibleto integrate the core cell 4 and extension cells 5, 6 in the manner of asemiconductor chip into the actual microrelay. In this way it ispossible to provide the protective circuit in the form of a singlecomponent or chip in the corresponding circuit. According to theinvention it is also possible to provide a quadripole module, in whichthe microrelay with its control inputs is connected either to the signaloutput, e.g. to the output signal of a switching stage, and the positivereference voltage or with the signal output and the negative referencevoltage. Thus, without component expenditure, it is possible to selectthe action direction.

[0070]FIG. 10 shows in a more detailed form the structure of aprotective circuit for protecting the non-replaceable fuse Fl of thestylized Z barrier, as described hereinbefore with the aid of a corecell and extension cells in the wiring with a microrelay. The structurewith a following fuse mechanism S, which is represented as a Zenerbarrier, and the following load L1 essentially corresponds to theapplicant's earlier-dated application WO 00/62394, but in whichconventional switching elements are used.

[0071] The protective circuit between the terminals 11, 12 and 18, 14shown in FIG. 10 essentially corresponds to the embodiment according toFIG. 8. Between the input terminal 11 and input terminal 12 is providedin series a resistor R2, downstream of which is connected a Zener diodeD1, as well as a further resistor R5. The emitter of a bipolartransistor Q2 is at the input 11 and its collector is located across aresistor R4 and a resistor R5 at the input terminal 12.

[0072] The base of transistor Q2 is placed across a feedback resistor R3at terminal 18. The base is simultaneously at the potential between theresistor R2 and the cathode of the Zener diode D1.

[0073] The device to be protected here comprises the load L1 and/or anupstream fuse mechanism S constructed as a Z barrier. For the protectionof the load L1 and Z barrier S a microrelay MR1 is provided between theterminals 11, 18 of switch 21. The input terminal 7 of microrelay MR1 islocated at input 11, whilst its other input 8 is at the collector oftransistor Q2 and resistor R4. Normally and with the standard supplyvoltage applied, the switch 21 of microrelay MR1 is closed, so that saidvoltage is supplied to the following fuse mechanism S.

[0074] The fuse mechanism S constructed as a Zener barrier is onlydiagrammatically shown in FIG. 10. In particular, the circuit shown isnot intended to constitute an approvable Zener barrier. In the presentcase the fuse mechanism S has in the serial path between the terminalpoints 18, 25 a resistor R7 with a following fuse Fl, which is designedas a blow-out fuse, together with a further resistor R6. At the nodalpoint 27 between fuse Fl and resistor R6 is located the cathode of aZener diode D3, whose anode is at terminal 12 or 14. For safety reasonsit is also possible to interconnect in a Zener barrier a plurality ofZener diodes.

[0075] The fuse mechanism S with the input terminals 18 and 14 and theoutput terminals 25 and 14 has in the embodiment according to FIG. 10the function of protecting the load L1 at terminals 25, 14 on the onehand against an overvoltage and on the other against an excessivecurrent flow.

[0076] The Zener diode D3 between terminals 27 and 14 is designed insuch a way that if an overvoltage occurs the terminal points 27, 14 arelimited to the Zener voltage. If an excessive current flow occursbetween points 18 and 25, there would be a disconnection or interruptionthrough the fuse Fl in the form of a blow-out fuse.

[0077] As the blow-out fuse Fl of the Zener barrier, particularly if thelatter is in the explosion-proof sector, must not be changed, butinstead following the response of F1 the entire Zener barrier wouldrequire replacement, a response of F1 should be avoided. To this end inthe embodiment according to FIG. 10 an upstream-connected protectivecircuit is provided on the basis of the-preceding arrangements accordingto FIGS. 1 to 9. The function of said upstream protective circuit,particularly in the case of an overload, but also with an excessivecurrent due to an overload, is to permit a faster disconnection of thevoltage applied, so that there is no need for the following fusemechanism S to respond and consequently its components can remainwithout being impaired and consequently remain in place for furthersafety.

[0078] The protective circuit according to FIG. 10 between the terminalpoints 11, 12 and 18, 14 consequently performs the following functionwith an overvoltage. In normal operation the switch 21 of microrelay MR1between points 17, 18 is closed. The transistor Q2 is blocked, so that acontrol voltage is applied to terminals 7, 8 of microrelay MR1 andswitch 21 is closed.

[0079] If an overvoltage now reaches the terminals 11, 12, there is acurrent flow across the Zener diode D1 and the resistors R5, R2 inseries therewith. Thus, there is a voltage potential at the base oftransistor Q2, which turns on the latter. The drive voltage at terminals7, 8 of microrelay MR1 is switched off, so that switch 21 is opened andconsequently the disconnection of the following fuse mechanism S andload L1 is brought about. In addition, the circuit also limits thevoltage applied to the input terminals 7, 8 of microrelay MR1, so thatthe latter is protected against an overvoltage.

[0080] The upstream-connected protective circuit with microrelaysconsequently allows in an advantageous manner the protection of thefollowing load, the fuse mechanism S and the load L1 againstovervoltages and excessive currents. In addition, disconnection isimproved by galvanic separation and the design possibilities for theupstream protective circuit, as shown in FIGS. 1 to 9, are significantlyextended.

1. Protective circuit (1) for limiting the voltage for a device to beprotected, having: in each case one input terminal (11) and one outputterminal (18, 20), a common line (12, 14) and a voltage detector (4)connected between the input terminal (11) and common line (12, 14), inwhich the device to be protected is located in an output circuit of theprotective circuit and is separable by means of a controllable switchingdevice provided in serial manner between the input terminal (11) andoutput terminal (18, 20) and in which the voltage detector is inoperative connection with the controllable switching device,characterized in that the switching device is a micromechanicalcomponent, which-has a control circuit with a high input resistance andan output circuit galvanically separated from the control circuit andswitchable via the latter, in which the output circuit in the openedswitching state has a galvanic separation of its output terminals (17,18, 19, 20).
 2. Protective circuit (1) according to claim 1,characterized in that the voltage detector (4) has an electronic switch(T1) and a voltage sensor (Dz), in which the control terminal of theelectronic switch (T1) is in electrical operative connection with thevoltage sensor (Dz) and the control circuit of the micromechanicalcomponent (MR1, MR2) is in electrical operative connection with theelectronic switch (T1).
 3. Protective circuit according to claim 2,characterized in that the voltage sensor (Dz) is a voltage sensor diode(Dz), particularly a Zener diode, and the electronic switch is atransistor (T1).
 4. Protective circuit according to claim 3,characterized in that the control circuit of the micromechanicalcomponent (MR1) is arranged in parallel to the collector-emitter path orto the source-drain path of the transistor (T1).
 5. Protective circuitaccording to one of the claims 2 to 4, characterized in that with thecontrollable switch (T1) is associated at least one operating resistor(Rc) and with the voltage sensor diode (Dz) at least one resistor (Rb)and that the control terminal of the controllable switch (T1) is inoperative connection with the resistor (Rb) and the voltage sensor diode(Dz).
 6. Protective circuit according to claim 5, characterized in thatthe control circuit of the micromechanical component (MR2) is parallelto the operating resistor (Rc).
 7. Protective circuit according to oneof the claims 4 to 6, characterized in that the control circuit of themicromechanical component (MR1) is connected to the base or gate and tothe collector or drain of the transistor (T1), respectively. 8.Protective circuit according to one of the claims 1 to 7, characterizedin that the voltage detector is provided in the form of a core cell (4)and there is a first adjusting device (5) between the core cell (4) andthe positive terminal (11) for the supply voltage and/or a secondadjusting device (6) between the core cell (4) and the negative terminal(12) for the supply voltage.
 9. Protective circuit according to one ofthe claims 8 to 11, characterized in that the adjusting device (5, 6)has at least one series resistor (Rv) or a series connection comprisinga series resistor (Rv) and a Zener diode (Dz).
 10. Protective circuitaccording to one of the claims 1 to 9, characterized in that themicromechanical component is constructed as a microrelay (MR) with anormally open contact function.
 11. Protective circuit according toclaim 10, characterized in that the control circuit (7, 8) of amicrorelay (MR1) constructed as a normally open contact is at theemitter-collector path of the transistor (T1).
 12. Protective circuitaccording to one of the claims 1 to 9, characterized in that themicromechanical component is constructed as a microrelay (MR) with anormally closed contact function.
 13. Protective circuit according toone of the claims 10 to 12, characterized in that the microrelay (MR1)with an output terminal (17) is directly or indirectly present at thepositive or negative supply voltage terminal (11, 12) and with the otherterminal (18) directly or indirectly at the device to be protected. 14.Protective circuit according to one of the claims 10 to 13,characterized in that the microrelay (MR1) detects the voltage at itsinput terminals (9, 10) and if an overvoltage occurs the microrelay(MR1) abruptly changes its switching state.
 15. Protective circuitaccording to one of the claims 10 to 14, characterized in thatelectrical components (4, 5, 6, 16) are connected upstream or inparallel with the microrelay (MR1) for voltage and/or current limitingor said electrical components are integrated in the microrelay (MR1).16. Protective circuit according to one of the claims 1 to 15,characterized in that the device to be protected is a load (LI) withupstream-connected voltage and current limiting device (S). 17.Protective circuit according to claim 16, characterized in that thevoltage and current limiting device (S) connected upstream of the load(L1) is a Zener barrier (D3).
 18. Protective circuit according to one ofthe claims 2 to 17, characterized in that the voltage sensor (Dz) isdesigned as a power sensor or as a temperature sensor.